On 6/17/22 06:55, Kevin Kofler via devel wrote: > Ben Cotton wrote (on behalf of Daan De Meyer, Davide Cavalca, Andrii > Nakryiko): > >> Fedora will add -fno-omit-frame-pointer to the default C/C++ >> compilation flags, which will improve the effectiveness of profiling >> and debugging tools. > […] >> Any performance or efficiency work relies on accurate profiling data. > > So, you propose to destroy programs' performance in order to allow people to > more easily improve programs' performance? That strikes me as a particularly > bad idea.
A much better solution would be to enhance perf to run the unwinder in user mode. The whole reason that this is even a tradeoff is because perf does unwinding in kernel mode, and a kernel-mode DWARF parser is an terrible idea for security reasons. > The profiling data collected with -fno-omit-frame-pointer will also not be > accurate for when the software is compiled with the default (under -O2 and > above) -fomit-frame-pointer. (In particular, the cost of shorter functions > will be significantly higher (in relation to the total) with the frame > pointer than with omitted frame pointer.) So it can actually lead to the > software getting tuned only for the Fedora build configuration. > > In addition, the frame pointer also increases code size (though, to be fair, > the asynchronous unwind tables that we usually want for code compiled with > -fomit-frame-pointer also take space). > >> Sampling profilers probe the target program's call stack at regular >> intervals and store the stack traces. If we collect enough of them, we >> can closely approximate the real cost of a library or function with >> minimal runtime overhead. > > Sampling profilers are fast, but will never reach the accuracy of Valgrind > (which, as far as I know, can deal with DWARF unwinding just fine). > > Kevin Kofler Valgrind is not helpful for profiling production workloads. It is too slow and will not provide an accurate indication of where the time is being spent. That requires a sampling profiler. -- Sincerely, Demi Marie Obenour (she/her/hers) _______________________________________________ devel mailing list -- devel@lists.fedoraproject.org To unsubscribe send an email to devel-le...@lists.fedoraproject.org Fedora Code of Conduct: https://docs.fedoraproject.org/en-US/project/code-of-conduct/ List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines List Archives: https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org Do not reply to spam on the list, report it: https://pagure.io/fedora-infrastructure
